1. Field of the Invention
The present invention relates to an organic electroluminescent device comprising a polymer layer containing a polyimide (PI) having a triphenylamine derivative (TPD) unit.
2. Description of the Related Art
An organic electroluminescent (EL) device is an active drive type emission display device that operates under the principle that when current is applied to fluorescent or phosphorescent organic compound layers (referred to as “organic layers” hereinafter), electrons and holes are combined in the organic layers to then emit light. Organic EL devices have various advantages including being lightweight, simple components, having a simplified fabrication process, and offering a wide viewing angle. Also, organic EL devices can enable moving picture display perfectly with high color purity, and the devices have electrical properties suitable for portable electronic devices at low power consumption and low driving voltage.
A general organic EL device is constructed such that an anode is formed on a substrate, a hole transport layer, an emissive layer, an electron transport layer, and a cathode are sequentially formed on the anode. Here, the hole transport layer, the emissive layer, and the electron transport layer are organic layers made of organic compounds. The electroluminescence mechanism of a general organic EL device will now be described. When a voltage is applied between an anode and a cathode, holes are moved from an anode to an emissive layer via a hole transport layer, and electrons are moved from a cathode to the emissive layer via an electron transport layer. The electrons and holes meet in the emissive layer for recombination, forming excitons. The excitons are subjected to radiative decay, producing light having a wavelength corresponding to a band gap of an emissive layer forming material.
To date, there have been made various attempts to enhance emission efficiency and lifetime characteristics of the organic EL device, including the followings: 1) facilitating injection of electrons and/or holes; 2) increasing electron and/or hole mobility; 3) facilitating recombination of electrons and holes; and 4) increasing photo conversion efficiency.
To achieve the above stated goals, the following techniques are provided: 1) minimizing an energy barrier between an electrode and a transport layer; 2) increasing electron and/or hole mobility through cascade transport; 3) inducing a balanced current using a hetero-junction structure or a low mobility emitter; and 4) using a phosphorescent material and fluorescent/phosphorescent hybrid materials as light-emitting layers.
Another method of enhancing emission efficiency and lifetime characteristics of the organic EL device which has an organic layer structure comprised of a hole transport layer, a light-emitting layer, and an electron transport layer is to control injection, movement and recombination of electrons and holes by providing an polymer interlayer between the hole transport layer and the light-emitting layer.
In other words, the polymer interlayer serves as a buffer layer that prevents migration of sulfur from the hole transport layer, usually a stacked layer of PEDOT/PSS(poly(3,4-ethylenedioxythiophen)/polystyrene sulfonate), and prevents a PEDOT/PSS layer, which is a strongly acidic layer, from directly contacting with the light-emitting layer. In addition, the polymer interlayer prevents electrons and exitons from being induced into the PEDOT/PSS layer, thereby enhancing life characteristics of the organic EL device.
Cho, et al. proposed a polymeric EL device having a hole blocking layer into which PMDA-ODA type polyimide (PI) is injected to various thicknesses in various packing densities, as described in Thin Solid Films 417(2002) 175-179, “The role of PI interlayer deposited by ionized cluster beam on the electroluminescence efficiency” between an ITO electrode and a light-emitting layer made of BEH-PPV polymer using ionized cluster beam (ICB). Here, the PMDA-ODA type polyimide serves as a hole injection barrier that prevents impurities of the ITO electrode from being diffused into the inside of the polyimide layer, causing balanced recombination of holes and electrons. The above-referenced document discloses a PI layer serving to reduce a hole tunneling probability, rather than serving as an electron blocking layer, in order to provide an organic EL device with enhanced efficiency using balanced charge carriers.
Kim, et al. proposed a polymeric EL device (PLED) having a thin polymer interlayer between an PEDOT/PSS and a light-emitting layer, as described in Applied Physic Letter 87(2005), 023506, “Spin-cast thin semiconducting polymer interlayer for improving device efficiency of polymer light-emitting diodes”. The thin interlayer blocks the radiative exitons from direct quenching by PEDOT-PSS and thus removes a nonradiative decay channel introduced by PEDOT-PSS. This exciton blocking property of conjugated polymer interlayer significantly improves the efficiency and lifetime of PLEDs. As a thin conjugated polymer interlayer, triphenylamine-based conjugated polymers, such as poly(9,9-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine) (PFB) and poly(9,9′-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) can be used, since such polymers are semiconductors having a wide band gap, low ionization potential, and high hole mobility due to their triphenylamine groups in the polymer backbone. However, these conjugated polymers are readily soluble in the common organic solvents, such as toluene and xylene, which are also used for light emitting polymer process. Therefore, when the light-emitting polymer was spin-coated, the underlying spin-cast polymer interlayer was partially dissolved and then washed out by the solution of light emitting polymer, or intermixed with light emitting polymer. The thickness control of a thin conjugated polymer interlayer is, thus, fundamentally difficult, which is a serious problem for the practical use.